Hydraulically assisted pilot operated valves are common in household appliances, such as in washing machines. These valves normally have a water inlet, a flexible sealing diaphragm and a water outlet. The flexible sealing diaphragm normally has some small apertures to permit the incoming water pressure to communicate with an upper chamber to force the valve closed. To permit the valve to open the diaphragm also has a slightly larger aperture communicating between the upper chamber and the valve discharge. This aperture is normally closed by a lightly spring loaded metallic plunger which is forced away from the aperture by means of an electromagnet to permit the valve to open.
Such a prior art valve is shown diagrammatically in FIG. 9a herewith. The water under pressure enters the inlet chamber marked "A" which communicates with the upper chamber marked "B" through the communicating holes 26 in the flexible sealing diaphragm 1 and its rigid diaphragm support 2. The bleed hole(or orifice) 15 to the valve discharge chamber marked "C", is normally sealed by the metallic plunger 34 which has an elastomeric sealing surface where it contacts the rigid diaphragm support 2.
As the bleed hole 15 from chamber "B" to chamber "C" is sealed the water pressure forces the diaphragm 1 and its support 2 to seal on the valve body 14 and thus prevents the flow of fluid through the valve.
When an electric current is passed through the solenoid 13 the metallic plunger 34 is drawn away from the diaphragm 1 and its support 2 to open the bleed hole 15. Since the bleed hole 15 is normally slightly larger in area than the communicating ports 26 the pressure in chamber "B" is reduced and thus the valve may open as shown in FIG. 9b.
When the electric current is cut off from the solenoid 13 the metallic plunger 34 once again seals the bleed hole 15 and the valve closes.
With this type of valve the valve requires a continuous supply of electric current to remain open.
With this type of valve the force which must be applied to the metallic plunger 34 to open the valve is the product of the water pressure and the area of the bleed hole 15. Typically the bleed hole diameter is 1 mm so at a water pressure of 1 MPa a force of 0.8 Newtons is required to open the valve.
The high forces to unseat the metallic plunger and the requirement of a continuous supply of electric current makes unsuitable for low voltage battery operated applications, such as garden irrigation systems, which are expected to operate for many months from a small dry cell battery.
Other known valves which are also not suitable for low voltage battery operated applications are disclosed in German Patent Application No. 3927611, International Patent Application Nos. PCT/GB92/00986 and PCT/US87/00214, Australian Patent Application Nos. 18576/88 and 55204/94 and United Kingdom Patent Application No.2149148.
An object of the invention is to overcome the limitations of the above mentioned valves by providing a valve which is suitable for use in low voltage battery operated applications.